Today we're going to try and settle the score between two words: it's and its. These two words sound the same, but can be confusing in writing. First, we'll clarify the meaning of the two words with a couple definitions:
Its without an apostrophe is the possessive version of the pronoun "it," as in:
That bunny is deadly. Its fangs are venomous!
It's is a contraction meaning "it is," as in:
It's no longer fashionable for a knight to say "Nih!"
As mentioned before, the two words sound identical. Context usually aids a listener to understand the speaker's meaning.
It's not just the auditory resemblance that causes the confusion. Punctuation more specifically the role of the apostrophe muddies the situation even further. Gordon Jarvie explains in the Bloomsbury Grammar Guide:
The apostrophe has two functions in punctuation. It marks the possessive case and it indicates contractions or the omission of letters in spelling certain words.
With "it's," we're working with the latter case. The apostrophe functions to signify the contraction of the words it and is.
When using the possessive form of the pronoun it, apostrophes aren't used. In fact, apostrophes aren't used for any possessive pronouns, because the words themselves indicate possession. Jarvie explains further:
The possessive pronouns are mine, yours, his hers, its, ours and theirs. Note that no apostrophe is used in writing a possessive pronoun.
If you're still not sure which version of "its/it's" to use, we've got some simple tricks to help you figure it out. Let's take the example of this sentence:
It's merely a flesh wound.
An easy way to remember when to use "it's" instead of "its," try substituting the word "it's" for the contraction spelled out it is:
It is merely a flesh wound.
As a test to see if the word is used in the possessive sense, substitute in the word "his" or "her" for "it's" and see if the sentence is still grammatically correct.
Her the flesh wound
That makes no sense whatsoever. In other cases, though, the word "its" works because you're talking about something that belongs to it. Let's try this example:
What distinguishes Limburger cheese is its very pungent aroma.
Now try substituting the word "his" for "its" to see if the word is possessive.
What distinguishes Limburger cheese is his very pungent aroma.
Recognizing of course that cheese doesn't have a gender, the sentence still makes linguistic sense, so the possessive case withouth the apostrophe is correct.
Sources: Bloomsbury Grammar Guide by Gordon Jarvie; The Elements of Style by William Strunk Jr. and E.B. White.
Music from this Episode: "Liberty Bell March" by John Philip Sousa.